Esters are valuable products in industry and are produced in a variety of ways. Some of the more important reactions to produce esters involve reacting (1) an acid and an alcohol, (2) an acid anhydride and an alcohol, (3) an acid chloride and an alcohol, (4) an acid and an unsaturated hydrocarbon such as an alkene or an acetylene, (5) an ester and an alcohol, (6) an ester and an add, and (7) two different esters. Esterification reactions producing esters are equilibrium limited and historical preparation techniques involved two sequential steps. The first step was the reaction step which ceased when equilibrium was reached. Generally, upon completion of the reaction, both unreacted reactants and the esterification products were all present in one mixture therefore necessitating a second step to separate the product of interest. The obvious drawbacks to the historical approach of producing esters are the cost of the two-step approach, often involving several reactors and separators, and the equilibrium-imposed limitation on the quantity of ester formed.
To overcome these drawbacks, some ester producers have used various techniques which allow the chemical reaction and the separation of the products to occur simultaneously. One such technique employed is reactive distillation. For example, U.S. Pat. No. 4,435,595 disclosed using reactive distillation to produce high purity methyl acetate from methanol and acetic acid. The reactive distillation process involved countercurrently flowing acetic acid and methanol through a single reactive distillation column in the presence of an acidic catalyst such as sulfuric acid. The acetic acid, in addition to being a reactant, also functioned as an extractive agent for the unreacted methanol and the produced water. Then the methyl acetate was separated from the acetic acid and continuously removed from the top of the column. The methanol was stripped from the water and the water was continuously removed from the bottom of the column. Using reactive distillation increased the extent of the reaction beyond equilibrium. One of the major drawbacks to this approach, however, is the cost of the reactive distillation column itseft. Since corrosive acids are used as catalysts, the column must be constructed out of materials able to withstand the harsh conditions. Such materials are generally expensive and over time will also corrode and need replacement.
Another technique which has been investigated and applied to numerous types of equilibrium-limited reactions in order to shift equilibria to favor the yield of products is the use of reactive chromatography. Reactive chromatography has been described as a technique employing a chromatographic system that is used both to convert one or more components and to simultaneously separate one or more of the products that are formed. Several different operating configurations such as a fixed bed with pressure swing or cylindrical annular bed with rotating feed input source, a countercurrent moving bed, and a countercurrent simulated moving bed have been explored. See generally, Vaporciyan, G. G.; Kadlec, R. H. AIChE J. 1987, 33 (8), 1334-1343; Fish, B. B.; Carr, R. W. Chem. Eng. Sci. 1989, 44, 1773-1783; and Cart, R. W. In Preparative and Production Scale Chromatography; Ganetsos, G., Barker, P. E., Eds.; Chromatographic Science Series Vol. 61; Marcel Dekker: New York, 1993; Chapter 18.
The esterification reaction of acetic acid with ethanol to form ethyl acetate and water and the simultaneous separation of the products has been accomplished in a fixed bed chromatographic reactor. See, Sardin, M.; Villermaux, J.; Nouv. J. Chim., 1979, 3(4), 255-261; and Sardin, M.; Schweich, D.; Villermaux, J. In Preparative and Production Scale Chromatography; Ganetsos, G., Barker, P. E., Eds.; Chromatographic Science Series Vol. 61; Marcel Dekker: New York, 1993; Chapter 20, pp 502-508. In these related references, the solids contained in the fixed bed were a mixture of activated alumina to effect the separation and a cation exchange resin in acidic form to catalyze the esterification reaction. The ethyl acetate product was not adsorbed by the bed while the water product was, thereby separating the two products. The system was operated in a pulsed regime and demonstrated a conversion greater than that available at equilibrium.
Other reactions and separations, such as mesitylene hydrogenation, have been accomplished using simulated moving beds. See, Ray, A.; Tonkovich, A. L.; Aris, R.; Carr R. W. Chem Eng. Sci., Vol 45 No 8 2431-2437 (1990). Some applications of simulated moving beds have focused on simultaneous reaction and catalyst regeneration. In U.S. Pat. Nos. 4,028,430 and 4,008,291 an alkylation reaction and catalyst regeneration through the removal of adsorbed water were disclosed. However, applicants are the first to realize that the simulated moving bed technique combined with reactive chromatography can be successfully applied to esterification reactions, and specifically to the esterification of an alcohol and a carboxylic acid to form an ester and water such as the esterification of methanol by acetic acid to form methyl acetate and water. Applying simulated moving bed technology to reactive chromatography for esterification will achieve high amounts of conversion with less process equipment as compared to fixed-bed systems. In addition, the disclosed invention eliminates costs associated with the recycle of unconverted reactants which are common in other processes.